There are positive displacement type compressors and centrifugal type compressors in the compressors used in refrigerators and air conditioners. In the displacement type compressors, there are reciprocating type, rotary type, scroll type, and screw type, in light of structure, in which, basically, a gas pressure is boosted by reducing volume of drawn gas.
The centrifugal compressor increases a moving speed of gas by rotating an impeller, and transforms a kinetic energy of the gas to a pressure in a diffuser fitted to an outlet of the impeller. In the reciprocating type compressors, there are connecting rod type, scotch yoke type, and plunger type depending on a mechanism that reciprocates the piston.
A related art multi-stage compressor will be explained, with reference to FIGS. 1 and 2. For obtaining a high pressure compressed gas, while an excessive temperature rise of the compressed gas is prevented, a multi-stage type compressor is employed, in general, three, or four stages, with four cylinders. FIGS. 1 and 2 illustrate related art three stage four cylinder compressor systems in related art multi-stage compressor systems, schematically.
Referring to the drawings, the three stage four cylinder compressor system is provided with two first stage compression cylinders 60, one-second stage compression cylinder 70, and one third stage compression cylinder 80.
Of the different stages of cylinders, the two first stage cylinders 60 may be arranged adjacently as shown in FIG. 1, or oppositely as shown in FIG. 2. In view of piston movement, the two first stage compression pistons 62 in the three stage compression system in FIG. 1 have a 90 degree phase difference of movement, and the two first stage compression pistons 62 in the three stage compression system in FIG. 2 have a 180 degree phase difference of movement. The second stage compression piston 72 and the third stage compression piston 82 are arranged at positions remained after the two first stage compression pistons 62 are arranged.
In the meantime, the three or four stage four cylinder compression system requires means for converting rotational movement transmitted from driving means, such as a motor, or an engine, to a linear reciprocating movement. Since a scotch yoke mechanism has been often used as such movement converting means in the related art, a related art scotch yoke type compressor will be explained, with reference to FIG. 3.
The related art scotch yoke type compressor is provided with in general a crank shaft 10, a crank pin 20, a sliding block 30, a frame 40, and one or more than one piston 50.
The crank shaft 10 is connected to a driving shaft of driving means (not shown), such as a motor, or an engine, and the crank pin 20 is connected to the crank shaft 10, for revolving along a fixed circular locus around the crank shaft 10 as the crank shaft 10 is rotated. The sliding block 30, rotatably coupled to the crank pin 20, moves on a fixed plane along a fixed locus as the crank pin 10 moves, and the frame 40, enclosing the sliding block 30 to impose a certain limitation to the movement of the sliding block 30, reciprocates in one direction according to the movement of the sliding block 30. The piston 50 fixed to the frame 40 draws and compresses a gas in a cylinder (not shown) as the frame 40 reciprocates.
A structure of the sliding block 30 fitted to an inside of the frame 40 will be explained in detail.
The frame 40 has one pair of parallel buffer plates 31 fitted to opposite two inside surfaces thereof, each supported on a plurality of springs 33 having both ends fixed to the inside surface thereof and the buffer plate 31. There is the sliding block 30 between the one pair of buffer plates 31, and there are a plurality of rolling needle bearings 35 between the sliding block 30 and each of the buffer plates 31. The sliding block 30 and the rolling needle bearings, and the buffer plate 31 and the rolling needle bearings 35 are pressed by the plurality of springs 33 by appropriate pressures to make close contact to each other. U.S. Pat. No. 5,846,059 discloses a structure of a scotch yoke similar to the foregoing structure.
The operation of the foregoing scotch yoke type compressor will be explained.
When the crank pin 20 is revolved along a fixed circular locus around the crank shaft 10 as the crank shaft 10 is rotated upon reception of a driving force from the driving means, the sliding block 30, rotatably coupled to the crank pin 20, also revolves around the crank shaft 10, too. Since rotation of the frame 40 is held back by the piston 50 having one end fixed to the frame 40 and the other end placed in the cylinder, rotation of the sliding block 10 is held back by the buffer plates 31 and the rolling needle bearing 35 fitted to the frame 40. Therefore, the sliding block 30 is revolved around the crank shaft 10 while maintaining a fixed angle with the frame 40.
As the sliding block 30 moves while maintaining a fixed angle with the frame 40, i.e., the sliding block 30 makes no rotational movement with respect to the frame, the movement of the sliding block 10 may be analyzed into a translational motion in a movement direction of the piston 50, and a translational motion in a direction parallel to the buffer plates 31. The translational motion in the direction parallel to the buffer plates 31 is not transmitted to the frame 40 by action of the rolling needle bearing 35. However, since the translational motion in the movement direction of the piston 50 is transmitted to the frame 40, the frame 40 reciprocates in the movement direction of the piston 50. One or more than one piston 50 fixed to an outside of the frame 40 reciprocates to draw or compress a gas as the frame 40 reciprocates. When two pistons 50 are fitted to outside of the frame 40 oppositely, movements of the two pistons 50 have a 180 degree phase difference.
If two scotch yokes are coupled to the same crank pin, with movement directions of the pistons fixed to each of the scotch yokes made different from one another by a predetermined angle, four pistons each making a reciprocating movement having a fixed phase difference from a reciprocating movement of an adjacent piston will be obtained. Accordingly, it is possible that one driving unit is made to drive four pistons at a time. Taking the problem of vibration into account, it is preferable the angle is 90 degrees, when movement of each of the pistons has a 90 degree phase difference from movement of the adjacent piston.
The foregoing related art multi-stage compressor has the following problems.
The three stage four cylinder compression system requires flow passages from the two first stage compression cylinders to the second stage compression cylinder, and from the second stage compression cylinder to the third stage compression cylinder inside of the compressor for providing the three stage compression, which requires a complicate inside structure, is difficult to fabricate, and costs high. The four stage four cylinder compression system has the same problem.
In the meantime, the structure of the scotch yoke used often in the related art multi-stage compressor has the following problems.
First, lubrication is required between the rolling needle bearings, the buffer plates, and the sliding block for assuring proper movements and appropriate wear, and cooling elements heated by friction. Therefore, it is required to apply grease to the parts, which is however leaks to contaminate other components, or mixed with the compressing gas.
Second, consequently, it is required to maintain air tightness for prevention of the grease leakage, which is, not only difficult, but also requires additional cost.
Third, appropriate setting, or selection of a pre-locking force, and a stiffness of the springs, is difficult.
If the pre-locking force is too small, the rolling needle bearings do not roll, to cause sliding friction, of which friction force is greater than the rolling friction force, that causes a poor efficiency. Opposite to this, if the pre-locking force is to great, putting excessively great force to the rolling needle bearings, defective operation and short lifetime of the rolling needle bearings are caused.
Moreover, if the spring is too soft, a position of the sliding block relative to the frame varies any time, resulting in a difficulty in maintaining a clearance between the piston and the cylinder at a proper level. Opposite to this, if the spring is too stiff, it is difficult to adjust a position of the frame for setting the clearance between the piston and the cylinder to the proper level, and, if an environmental temperature rises, an excessive force is applied to the buffer plates and the rolling needle bearing due to thermal expansion of the springs.